1. Field of the Invention
This invention relates to an improved drum brake device used for braking a vehicle. The invention provides an inexpensive mechanism for preventing an interference between a shoe return spring and an adjustable strut which maintains the spacing between a pair of brake shoes.
2. Description of the Related Art
FIG. 5 (Prior Art) shows a conventional drum brake for braking a vehicle. The conventional drum brake comprises: a brake drum (not shown) which rotates along with a wheel; a backing plate 1 fixed on a suspension device; a pair of brake shoes 3a and 3b which are movably supported by the backing plate 1, the brake shoes being allowed to move pivotally about an anchor 2 fixed to one end of the backing plate 1 (lower end as viewed in FIG. 5); a wheel cylinder 5 fixed to the other end of the backing plate 1 (upper end as viewed in FIG. 5), the wheel cylinder 5 being adapted to expand the pair of brake shoes 3a and 3b to an operating position whereby friction linings 4 attached to the brake shoes 3a and 3b are frictionally engaged with the inner peripheral surface of the brake drum when pressurized fluid is supplied to the wheel cylinder; an adjustable strut 7 disposed between the brake shoes 3a and 3b to maintain the spacing therebetween; and a return spring 6 provided adjacent the strut 7 for retracting the brake shoes 3a and 3b from the brake drum.
The strut 7 can freely extend or shorten its length by means of a screw device provided therein which is operated by rotation of an adjuster gear 11 provided in a middle portion of the strut 7. In addition, both ends of the strut are respectively pressed flat and are formed substantially bifurcate-shaped in section. One end portion of the strut 7 (left end as viewed in FIG. 5) holds both sides of a web 9a of the brake shoe 3a (left side as viewed in FIG. 5). One end portion of a parking lever 8 is pivotally supported by a pin 10 provided at one end of a web 9b of the other brake shoe 3b (right side as viewed in FIG. 5). The other end portion of the strut 7 holds a middle portion of the parking lever 8.
Further, the parking lever 8 and one end of an L-shaped adjuster lever 12 are pivotally supported by the pin 10. A middle portion of the adjuster lever 12 is engaged with the end portion of the strut 7 and a pawl portion 12a provided on the other end portion of the adjuster lever 12 is engaged with the adjuster gear 11 in such a manner that the adjuster gear 11 can be rotated by the pawl portion 12a. With respect to the return spring 6, one end portion thereof is abutted against the web 9a and the other end thereof is abutted against the middle portion of the adjuster lever 12 located between the pin 10 and the engaging portion of the strut 7, whereby the return spring 6 has two functions. The first function is to allow the adjuster lever 12 to rotate pivotally around the pin 10 when the braking force is operated, and the second function is to retract the pair of brake shoes 3a and 3b to their original position when the braking force is released. A drum brake device of this type is disclosed in detail in U.S. Pat. No. 4,101,011.
When the braking force is operated by the drum brake thus constructed, the single ends of the pair of brake shoes 3a and 3b (upper end as viewed in FIG. 5) are expanded away from each other by the wheel cylinder 5 fixed to the backing plate 1. Both brake shoes 3a and 3b are displaced toward the outside of the backing plate 1, pivoting around the anchor 2. The outer peripheral surfaces of the friction linings 4 of the brake shoes 3a and 3b are frictionally engaged with the inner peripheral surfaces of the brake drum, thereby causing a braking force to be produced by the friction between these surfaces. When the supply of pressurized fluid to the wheel cylinder 5 is interrupted, the brake shoes 3a and 3b are retracted to their original position by the return spring 6, thereby disengaging the outer peripheral surfaces of the friction linings 4 from the inner peripheral surfaces of tho brake drum.
When the parking brake is operated, the other end (lower end as viewed in FIG. 5) of the parking lever 8 is pulled toward the left in FIG. 5 by a cable (not shown), thereby causing the parking lever 8 to pivot clockwise around the pin 10 as viewed in FIG. 5. Since one end of the strut 7 abuts against the middle of the parking lever 8, this causes the strut 7 to be pushed toward the left as viewed in FIG. 5, thereby pushing the brake shoe 3a against which the other end of the strut 7 abuts, outward. Thus, the outer peripheral surface of the brake lining 4 of the brake shoe 3a is pushed against the inner peripheral surface of the brake drum. Simultaneously, as a reaction, the pin 10 supporting one end of the parking lever 8 is displaced outwardly, thereby pushing the brake shoe 3b to which the pin 10 is attached, outwardly. This pushes the outer peripheral surface of the friction lining 4 of the brake shoe 3b against the inner peripheral surface of the brake drum.
As a result, the outer peripheral surfaces of the linings 4 of both brake shoes 3a and 3b are pressed against the inner peripheral surface of the brake drum so that a braking force is obtained by the friction between the surfaces.
As the linings 4 are worn by repeated braking, the gap between the outer peripheral surfaces of the linings 4 and the inner peripheral surface of the brake drum is maintained within a prescribed range by causing the adjuster gear 11 provided on the strut 7 to rotate by the pawl portion 12a of the adjuster lever 12, thereby expanding the strut 7.
In a drum brake thus constructed and operated, it is inevitable that the return spring 6 and the strut 7 will interfere with each other, as best seen in FIG. 8 (prior art) because there is not enough room therebetween. For this reason, if any measures to meet this situation are not taken, vibration and expansion and contraction of the return spring 6 corresponding to braking will cause the return spring 6 and strut 7 to rub against each other, resulting in interfered operation of the components, undesirable noise and driver discomfort.
Therefore, the conventional drum brake is constructed as shown in FIGS. 6 (Prior Art) and 7 (Prior Art) so that interference between the return spring 6 and strut 7 can be prevented. In other words, in a drum brake as shown in FIG. 6 (Prior Art), an outer diameter of the return spring 6 is formed thick at its end portions and thin at its middle portion, thereby preventing the middle portions of the return spring 6 and the middle portion of the strut 7 from interfering with each other. This prevents undesirable sound from occurring regardless of vibration or operation of the drum brake.
In a drum brake as shown in FIG. 7 (Prior Art), a spacer 13 is held between the end of the return spring 6 and the web 9a of the brake shoe 3a, thereby preventing the middle portion of the return spring 6 and the strut 7 from interfering with each other.
However, the conventional drum brakes described above are troublesome and expensive to manufacture because of having to form only the end portions of the spring 6 thick. It is also difficult to provide and hold the spacer between the end portion of the return spring 6 and the web 9a.